1. Field of the Invention
This invention relates generally to apparatuses for processing semiconductor substrates and, more particularly, to thermal reactors and methods for controlling the temperature of such reactors.
2. Description of the Related Art
Reactors which can process a substrate while suspending the substrate without directly mechanically contacting the substrate, e.g., by floating the substrate on gas cushions, have relatively recently been developed for semiconductor processing. These reactors may be called floating substrate reactors. Such a reactor is commercially available under the trade name Levitor® from ASM International N.V. of Bilthoven, The Netherlands.
In the Levitor® reactor, which is also described in U.S. Pat. No. 6,183,565 B1, the entire disclosure of which is incorporated by reference, a substrate is supported by two opposite gas flows emanating from two heated and relatively massive reactor blocks located on opposite sides of the substrate. Each reactor block typically has a heated furnace body for heating the substrate. A small gap of less than about 1 mm is typically maintained between each furnace body and a corresponding substrate surface. The small gap results in a rapid heat transfer from the reactor blocks to the substrate by conduction through the gas when the substrate is processed, e.g., during a heat treatment, or exposure to elevated temperatures. An advantage of reactors such as the Levitor® reactor is that the relatively massive reactor blocks of the reactor act as thermal “fly-wheels,” resulting in a relatively stable temperature and reproducible performance.
Floating substrate reactors and methods for using such reactors for successive heat treatment of a series of planar substrates, one by one, is described in U.S. Pat. No. 6,746,237, issued Jun. 8, 2004. In those methods, the furnace bodies of the reactor blocks are typically continuously heated. After the furnace bodies have reached a desired temperature, a relatively cold substrate is placed for heat treatment in the vicinity of the furnace bodies. The typically colder substrate will withdraw heat from the furnace bodies and cause the temperature of the parts of the furnace bodies close to the substrate to decrease. The substrate is then heat treated. Because of the heat treatment time is typically relatively short in comparison to the thermal recovery time of the furnace body, the substrate is removed from the vicinity of the furnace body before the temperature of the continuously heated furnace body rises to the desired temperature again. After the temperature of the boundary surface rises to the desired temperature, another substrate is placed in the vicinity of the furnace body for heat treatment.
The methods described in U.S. Pat. No. 6,746,237 achieve a reproducible heat treatment of each of the substrates. The furnace body is at a desired temperature when a substrate is positioned for heat treatment. The substrate is removed from the vicinity of the furnace body before the temperature of the furnace body has recovered, and then the furnace body reaches the desired temperature again before positioning the next substrate in the vicinity of the furnace body for heat treatment. Thus, each substrate experiences roughly the same profile of heat treatment temperatures over time during the course of the heat treatment.
The substrates to be processed are typically provided in batches. As noted above, the ability to load successive substrates into a reactor at particular times allows a reproducible heat treatment profile to be set up. After all the substrates in a batch are processed, another batch is typically positioned for processing substrates in the reactor. This typically causes the reactor to be idle for an extended time period. Because the temperature of the reactor can drift in the time needed to position a new batch of substrates and to load a substrate from that batch into the reactor, the reactor typically must be re-initialized and set at the proper temperature before the next batch of substrates can be processed. Because of the high thermal capacity of the furnace bodies, the temperature of the reactor changes slowly and this initialization procedure can be time-consuming. Thus, the time needed for a reactor to reach the desired thermal conditions between batches can undesirably reduce the throughput of the reactor.
While described in the context of the Levitor® floating substrate reactor, the skilled artisan will appreciate that issues created by the thermal load of cold substrates can arise in other types of reactors, including a variety of hot wall reactors and/or reactors wherein a substrate is mechanically supported on a relatively massive susceptor. Furthermore, temperature overshoot issues are known to arise in many rapid thermal processing (RTP) contexts.
Accordingly, there exists a need for apparatuses and methods of controlling the temperature of a reactor to, e.g., allow for increased throughput.